SOLAR WATER HEATING
Hot water is required for many purposes and the sun can be used effectively, efficiently and economically to provide this heat. The warming effect of solar radiation is obvious and it is well known that a container of cold water, left exposed to the sun will be raised in temperature. Solar water heating systems are designed to make convenient use of this phenomenon. Solar water heaters generally employ a solar collector and a storage tank. The collector is by far the most widespread solar energy conversion device and there are several million in use around the world, of which designs vary from very simple to complex. The extent to which construction and manufacture is easily achievable varies around the world but ther e are some encouraging examples of innovations and government schemes which have greatly increased the application of solar water heating, not only in affluent areas but also in poorer, less developed and often rural areas.
The availability of solar energy
Please refer to the technical brief “Solar Thermal Energy” for details on the availability and varaiation of solar energy around the world. Insert link here….?
Passive vs. Active
Solar thermal systems in their simplest form consist of a solar collector and a storage tank. These systems are termed passive, whilst those that contain circulating pumps are known as active systems. (Solar Water Heater Industry in China, 2010)
Passive systems rely on the natural buoyancy or thermosyphon effect created when the
temperature of the water in the collector rises, causing the water itself to rise as it becomes less dense with increased temperature, thus inducing circulation in the circuit.
No electricity required for operation Minimal running costs Maintenance costs reduced as no mechanical parts
Reduced flexibility as collector has to be located beneath the solar store Less reliable as inconsistent rate of circulation
Table 1: Advantages and Disadvantages of Passive Solar Water Heating Systems
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Solar water heating
Storage Storage Tank
Flat plate collector
Figure 1: Passive and Direct Solar Thermal System (Illustration by Author, with reference to CIBSE Solar Heating Design and Installation Guide, 2007)
Active systems contain circulation pumps to induce movement of the water or heat transfer medium around the circuit.
Greater flexibility in design and location of system offered by integration of circulation pumps Greater efficiency induced through consistent circulation of water/heat transfer liquid Running costs of electricity used to operate pumps could be reduced through installation of renewable electricity power source, e.g. solar photovoltaics
Higher maintenance costs as mechanical parts are present in the system Running costs higher as electricity required for pump operation Reliable source of electricity required for operation
Table 2: Advantages and Disadvantages of Active Solar Water Heating Systems
Solar water heating
Flat plate collector Circulation Circulation pump
Figure 2: Active and Direct Solar Thermal System (Illustration by Author, with reference made to CIBSE Solar Heating Design and Installation Guide, 2007)
Direct vs. Indirect
Solar water heating systems can function as either direct or indirect. In a direct system, the heat transfer medium is water which is heated up and used directly from the collector/storage tank. In an indirect system, the heat transfer medium is not necessarily water and does not come into contact with the water that is used after being heated up through som e form of heat exchange system. The following are disadvantages inherent in direct systems. Scalding If the water has not been moving through the collector for a while and therefore becomes stagnant, upon using it again, the water may be at a dangerously high temperature by the time it leaves the point of use, i.e. a tap or shower. This could damage the equipment or block safety vents. If circulation is reduced, the system becomes blocked or there is a build-up of bacteria, there is a greater risk of the accumulation of silt, lime-scale and other debris. If there is a build-up of debris, the quality of the domestic hot water is likely to decrease. Another reason for this would be the water coming into contact with certain fittings and materials when the water in the system is stagnant and temperatures and pressures are high There is a risk that hot water could flow backwards round the circuit into the cold water cistern. A reduction in collector efficiency occurs if the return temperature at the base of the solar storage vessel is increased due to the circulation pumps in the system.
Fluid freezing Deposits Loss of water quality
Backflow/thermosiphoning Stratification disturbance
As well as minimising and/or avoiding the occurrence of the above, another advantage to installing an indirect system is the capability to introduce certain performance enhancers to the heat transfer medium without compromising the health of those using the domestic hot water generated. These enhancements may be in the form of antifreeze or corrosion inhibitors, which reduce the risk of damage to the equipment during colder months and the frequency of part replacement and maintenance.
Solar water heating
These systems are often designed with a storage tank and/or alternative, complementary heat source. With the latter, a system of pre-heating the liquid is employed to ensure the water reaches a temperature safe for domestic use, particularly with regard to the risk of legionella (See Section “Health and Safety – Things to be aware of” for further details). Pre-heating the liquid is achieved either by an instantaneous water heater or the collector itself. (CIBSE Solar Heating Design and Installation Guide, 2007)
Connecting to an existing domestic hot water system
Connecting a solar hot water heating system to a conventional hot water storage vessel raises its own problems, such as a reduced dedicated solar storage capacity or a loss of the ability to operate the legionella control method of incorporating back-up heating appliances into the system design. (See Section “Health and Safety – Things to be aware of” for more information
Selecting and sizing a solar thermal installation
Although the configuration of the system, i.e. whether it would be classified as direct or indirect, has an effect on the efficiency of a system, the collector itself generally has a greater influence. The two main types of solar collector are flat plate and evacuated tube. Flat plate
These tend to be cheaper but less efficient.
In their simplest form, these collectors are a rectangular frame with a row of pipes connected at the top and bottom. Cold water enters the system of pipework at the bottom and travels up and down and across, whilst sandwiched between an absorber and a plate of glass/plastic to then leave the system at the top of the frame on the opposite side to the inlet.
Hot water leaves network of pipes
Network of pipes through which water passes to heat up Absorber Insulation Cold water enters network of pipes
Figure 3: Section through a flat plate solar collector (Illustration by Author with reference made to CIBSE Solar Heating Design and Installation Guide, 2007 & Capturing Solar Energy, 2009)
Solar water heating
These tend to be more efficient but more expensive A series of pipes are connected at the top by a manifold. The tubes consist of a vacuum with a pipe running through the middle containing the working fluid. The water moves up and down and along through the series of pipes to exit the system at a significantly higher temperature.
Manifold casing Manifold
Fluid moves along manifold by passing up and down each tube
End of outer tube sealed
Evacuated Evacuated tube
Figure 4: Sections through an evacuated tube solar collector (Illustration by Author with reference made to CIBSE Solar Heating Design and Installation Guide, 2007 & Capturing Solar Energy, 2009) An important feature of both of the above is the medium through which the solar radiation is absorbed, i.e. the ‘absorber’. It is well known that dark surfaces with a matt finish absorb radiation more effectively than light and polished, therefore the absorbers incorporated into the design of these collectors are often painted/coated in a dark, matt finish. (Capturing Solar Energy, 2009) There are four main types of flat plate collectors:
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